1. Technical Field of the Invention
The present invention relates to a semiconductor integrated circuit device and, more particularly, to a technique which may be effectively applied to a semiconductor integrated circuit device having a bipolar transistor or/and a MISFET.
2. Prior Art
A technique which is most suitable for achieving high integration and high-speed operation of bipolar transistors is disclosed in "NIKKEI MICRODEVICES", NIKKEI McGrow Hill, November 1985, pp. 73-74. The following is a summary of the bipolar transistor producing method described in the above-mentioned literature.
First, a silicon nitride film is formed on a principal surface of an n-type epitaxial layer within a bipolar transistor forming region which is defined by an insulating film for isolation.
Next, a polycrystalline silicon film is formed on the silicon nitride film. Thereafter, the polycrystalline silicon film is patterned so that a region for forming an active-base region and an emitter region is opened, thus forming a base electrode.
Next, boron(B), which is a p-type impurity, is introduced into the base electrode. Thereafter, the surface of the base electrode is oxidized to form a silicon oxide film using as an oxidation-resistant mask the silicon nitride film which is exposed through the region for forming an active-base region and an emitter region.
Next, with the silicon oxide film on the surface of the base electrode used as an etching-resistant mask, the silicon nitride film on the region for forming an active-base region and an emitter region is removed by etching, and the silicon nitride film underneath the end portion of the base electrode on the side thereof which is closer to the removed region is removed by side-etching to form an undercut portion.
Next, a polycrystalline silicon film is deposited on the whole surface of the substrate in such a manner that the undercut portion is filled with the polycrystalline silicon film. Thereafter, the polycrystalline silicon film deposited on the flat portions except for that part of the silicon film which is buried in the undercut portion is removed by an anisotropic etching process such as reactive ion etching (hereinafter referred to as "RIE") to expose the surface of the epitaxial layer within the region for forming an active-baser region and an emitter region.
Next, thermal oxidation is carried out to form a silicon oxide film on a part of the polycrystalline silicon film buried in the undercut portion and on the exposed surface of the epitaxial layer.
Next, a p-type impurity is introduced into the principal surface region of the epitaxial layer within the region which is defined by the base electrode to form a p-type active-base region. An external base region is formed by diffusion of the p-type impurity in the base electrode into the principal surface region of the epitaxial layer through the polycrystalline silicon film buried in the undercut portion. The active-base region is connected to the external base region.
Next, a silicon oxide film and a polycrystalline silicon film are successively deposited on the whole surface of the substrate and then these films are removed by an anisotropic etching process such as RIE to form an emitter opening within the region defined by the base electrode.
Next, a polycrystalline silicon film is formed in such a manner as to connect with the active-base region through the emitter opening. This polycrystalline silicon film is then subjected to predetermined patterning to form an emitter electrode. An n-type impurity is introduced into the emitter electrode and then diffused into the active-based region to form an n-type emitter region.
In the bipolar transistor fabricated in this way, the external base region, active-base region, emitter region and emitter electrode can be formed in self-alignment with respect to the base electrode and it is therefore possible to achieve high integration. Further, since a region into which the p-type impurity in the base region is diffused is determined by both the amount in which the silicon nitride film underneath the base electrode is removed by side-etching to form the undercut portion and the amount in which a part of the polycrystalline silicon film buried in the undercut portion is oxidized, it is possible to form a smaller external base region than in the case where the bipolar transistor is formed by photolithography technique and it is therefore possible to achieve high integration.